The Crew 2

Here is a list of sub-100k vehicles, along with their category and cost in bucks, sorted from the cheapest:
SR 20,300 1957 Cadillac Eldorado Brougham
SR 20,300 2012 Jeep Wrangler
SR 21,700 2010 Chevrolet Silverado 1500
SR 39,200 2015 KTM 450 EXC Street Race Edition
SR 41,300 2015 Kawasaki KX450F Street Race Edition
JS 44,800 2017 Proto Hornet
SR 48,300 2013 Mini Cooper S Countryman All4
MT 49,700 2010 Chevrolet Silverado 1500 Monster Truck Edition
SR 51,100 2017 Harley-Davidson Iron 883
SR 51,800 2010 Ford F-150 SVT Raptor
SR 51,800 2008 Abarth 500
SR 53,900 2015 Indian Scout
SR 56,700 2015 Indian Chief Dark Horse
SR 57,400 2017 Harley-Davidson Street Glide
SR 60,200 2006 Hummer H1 Alpha
SR 60,200 1970 Nissan Fairlady Z 432 (PS30)
MT 70,000 2008 Abarth 500 Monster Truck Edition
MT 70,000 2010 Chevrolet Camaro SS Monster Truck Edition
JS 76,300 2017 Proto Brawler
MT 79,800 2012 Dodge Challenger SRT-8 392 Monster Truck Edition
MT 84,700 2011 Ford Mustang GT Monster Truck Edition
MT 90,300 2012 Jeep Grand Cherokee SRT 8 Monster Truck Edition
SR 93,800 2014 BMW R1200GS Adventure
SR 93,800 2012 Cadillac Escalade
MX 98,700 2015 KTM 450 EXC

There is no need to rush this achievement though. When you purchase vehicles necessary for all the competition categories and obtain all free trophy cars, there are only about 10 vehicles to go.

Season pass cars DO count towards this total. If you invest in that, you'll receive a large boost on the way to 30 cars. I believe the game will register your ownership of the cars after you do a few of the beginner races.